The present invention concerns a method for detecting islanding of DC/AC converters operating in grid connected mode and related DC/AC converter apparatus.
Grid connected power conversion and generation systems are adapted to generate electrical power for direct delivery to the power grid, in particular converting electrical power generated by renewable sources, such as, for instance, photovoltaic panels and wind turbines. Said power conversion and generation systems usually comprise an inverter apparatus adapted to convert a DC input voltage into an AC output voltage characterized by amplitude and frequency as required by the power grid specifications. Said inverters are generally adapted to be connected directly to the power grid and typically comprise a semiconductor H-bridge circuit working as a DC-AC converter.
Grid-connected power generation units need to comply with safety standards and regulations to be certified safe to connect to the public power grid. In particular, grid-connected systems need to be safe against islanding, the condition in which the power generation unit continues to power a location even though electrical grid power from the electric utility is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent automatic re-connection of devices. For that reason, power generation units must be adapted to detect islanding conditions and immediately stop producing power. Power generation units need therefore to be provided with anti-islanding capabilities that force the disconnection of the output of the DC/AC converter from the AC mains in case the mains voltage is missing.
This requirement can be fulfilled if the inverter of the power generation unit is provided with means to detect islanding and is connected to the grid via a suitable automatic switching circuit, preventing unintentional connection to a sub-grid or to a stand-alone grid and therefore preventing islanding conditions to happen.
Today there are various methods available to detect islanding and avoid non-safe operation in islanding mode for grid connected power systems applications, said methods being generally divided in passive and active methods.
Passive methods monitor parameters such as voltage and frequency, the grid voltage change rate or the grid frequency change rate and prevent the supplying of power from the inverter to the grid if the detected working conditions differ from the nominal ones. Examples of state of the art passive methods are based on the detection and analysis of voltage phase jump, rate of change of frequency, rate of change of voltage, current harmonics.
Active methods generally introduce disturbances to the connected circuit and then monitor the response to determine if the utility grid is still connected; some active methods are based on the communications between the inverter and the utility and are adapted to stop the inverter from supplying power to the grid when emergency occurs.
Examples of state of the art active methods are based on detection and analysis of impedance, frequency shift, frequency drift, Variation of Active Power and/or Reactive Power, Frequency Jump, Sandia Frequency Shift, Sandia Voltage Shift. Active methods are generally preferred by the inverter manufacturers because have lower NDZ (Non Detection Zones) and faster detection times compared to passive methods.
State of the art methods for detecting island conditions during inverter operation are generally affected by drawbacks concerning the immunity to frequency variations when grid connected, the stability of the system, the preservation of the power quality, the synchronization of the perturbation when there are multiple inverters in a plant.
In detail, the operation of one of the most common state of the art methods for detecting island conditions, based on the periodic injection of reactive power, is hereby described. This method is based on the idea of attempting to shift the frequency of the voltage at the point of common coupling (PCC), VPCC, by injecting reactive power on a time interval.
When the grid utility is connected, the frequency will not shift because the reactive power in the system is such that:Qload=Qinv+ΔQ 
Thus, when Qinv changes, ΔQ changes accordingly in order to satisfy the reactive power Qload requested by the load.
When grid is disconnected, ΔQ is not available anymore to balance the load request, therefore:Qload=Qinv 
The only way to preserve the balance condition when Qinv changes is therefore a frequency change since:
      Q    load    =                    (                  V          PCC                )            2        ·          (                        1                      ω            ⁢                                                  ⁢            L                          -                  ω          ⁢                                          ⁢          C                    )      
Thus, whenever the inverter detects a frequency shift in response to an injection of reactive power an island condition is detected.
The above method is characterized by a few drawbacks namely the lack of immunity to frequency variations when grid is connected, the synchronization between multiple connected inverters and the high level of perturbation introduced in the system.
Other state of the art methods for detecting island conditions during inverter operation make use of positive feedback control loops in order to start a frequency drift in a test signal. These methods usually show stability problems for instance when a weak utility is connected to the inverter, then possible instability in the power output can cause transients in the system and the positive feedback control loop to detect islanding can amplify and increase that instability.
It is therefore an object of the present invention to introduce a new method for the reliable detection and management of islanding conditions in grid connected inverters to overcome the state-of-the-art methods drawbacks.
In particular, it is an object of the present invention to introduce a new method, a related new AC/DC converter and a new power generation system comprising said AC/DC converter, for the reliable detection and management of islanding conditions regarding multiple grid connected inverters of the same plant.
The method according to the present invention is further adapted to optimize performance in terms of preservation of the power quality; provide synchronized perturbation for all the inverters of a plant; provide inverter shut down capability within the time requested by utilities and safety standards; provide immunity to grid frequency fluctuations.